It is proposed to compare and contrast the development of normal full-term infants, uncomplicated prematures, and full-term infants with trisomy 21 syndrome, by means of quantitative as well as impressionistic evaluations of intensive longitudinally gathered EEG and evoked potential data during the first year post-term. The primary purposes are to determine whether the experimental groups show lags and/or advancements in rates of development of the electrophysiological variables, or other deviations, and to determine the optimal time during the early weeks of life for observing specific differences. Correlative aims include (1) evaluation of the possible usefulness of interhemispheric amplitude symmetry as a developmental and/or clinical variable in the neonate, (2) further to investigate frequency characteristics of the newborn's EEG, (3) to study the development of interhemispheric synchrony in the infant, (4) to study the stimulus intensity discrimination capacity of the newborn brain, and (5) to study the frequency discrimination capacity of the newborn brain. Recordings will be made weekly during the first 10 weeks post-term (also pre-term in the prematures), and then at 3, 6, 9, and 12 months in 10 carefully selected subjects in each of the 3 groups. Additional single recordings will be obtained from appropriate subjects for the correlative studies. Basic quantitative longitudinal data on the early ontogenesis of human brain electrophysiological phenomena will be obtained, and will permit cross-validation of a number of reported observations, some of which may prove to be of clinical prognostic significance. Both the techniques used and the information obtained will be widely applicable in the other studies of infant development, especially in studies of other groups of at-risk infants, as well as in older children.